A scattered light smoke detector of the open type having a planar circuit substrate is known from EP 2093734 A1. Arranged on said circuit substrate are a light emitter for emitting an illumination light and also a light receiver mounted adjacent thereto for receiving measured light. The measured light results from a backscattering of the illumination light from for example smoke in a detection space located in the open air outside of the scattered light smoke detector. The light emitter emits the illumination light at right angles to the plane of the circuit substrate. The illumination light is at least partially scattered back in the detection space by approximately 180°, i.e. by between 170° and 190°. The backscattered light then reaches the light receiver as measured light, the main receiving direction of the light receiver likewise being at right angles to the plane of the circuit substrate.
A disadvantageous aspect in the case of said backward-scattered light array is that by far the greatest part of the measured light gets lost. Furthermore, the light receiver has no protection against environmental influences and mechanical effects.
EP 1191496 A1 and EP 1039426 A2 each disclose a scattered light smoke detector of the open type having a light emitter and a light receiver which are arranged in such a way that the scattering point of the light emitter and the light receiver is located outside of the scattered light smoke detector in the open air. Light emitter and light receiver are arranged in a scattered light array at a scattered light angle of approximately 80° to 90°. The main emitting direction of the light emitter and the main receiving direction of the light receiver are therefore inclined relative to one another by this approximately 80° to 90°. When such a scattered light smoke detector is fitted to a mounting surface, typically to the ceiling, the main emitting direction of the light emitter and the main receiving direction of the light receiver are then inclined by approximately 40° to 45° relative to said mounting surface.
As a result of the inclined scattered light array in relation to the mounting surface, the two scattered light smoke detectors of the open type have a comparatively large overall height.